Refrigeration using a high-pressure jet of steam is typically referred to as steam jet cooling. In this method, the cooling system includes a source of steam, an ejector and a closed water vessel fluidly connected to the ejector and containing a refrigerant, usually water. In use, the steam is passed through the ejector to create a partial vacuum in the closed water vessel. Some of the water in the closed vessel vaporizes at the low pressure of the partial vacuum and exhausts into a chamber of the ejector. The vaporized water absorbs heat from the water that remains liquid in the vessel, thereby cooling the liquid water by evaporative cooling. The chilled water is pumped through the system to cool air, and the vaporized water in the ejector is directed to a condenser, where it condenses into liquid and returned to the cooling system. Variations of the aforementioned steam jet system will be known to those of ordinary skill in the art. Systems of this type could also be used for heating.
In order to be commercially practical, an ejector type heat pump needs to have a coefficient of performance (COP) of 1.0-or-greater even when condensing temperatures exceed 100 degrees Fahrenheit. As is known, COP is the ratio of the cooling power to the input power required to achieve the cooling. In an ejector type heat pump the COP is determined by the entrainment ratio (the mass ratio of the refrigerant fluid to working fluid), and the ratio of the enthalpy change of the refrigerant fluid to the enthalpy change of the working fluid. The COP also correlates to the (“lift”) ratio of the pressure of the fluid leaving the ejector to the stagnation pressure of the refrigerant entering the ejector. The lift ratio, particularly at high ambient temperatures, requires a high pressure at the exit of the ejector in order for the vapors to reach their saturation pressures and to condense. This requires a substantial amount of heat energy to be supplied to the working fluid, which increases the enthalpy change of the primary fluid and therefore reduces the system's efficiency. In fact, a major disadvantage with the conventional steam jet cooling system is the low coefficient of performance (COP), which is typically about 0.2 to 0.3. One method to improve the COP of an ejector system is to choose a refrigerant fluid (sometimes referred to as the secondary fluid) that is different from the working fluid (sometimes referred to as the primary or driving fluid). Such two-fluid jet cooling systems have achieved COPs of up to 0.5 but have not found commercial acceptance. Another problem with conventional steam jet cooling system is the use of non-environmentally friendly fluids as the working fluid. For example, perfluorocarbon has been used as the primary fluid because of its high molecular weight and immiscibility with common refrigerants such as water, acetone, ammonia, and methanol. However, perfluorocarbons have high global warming potentials. Moreover, typical ejector systems suffer large efficiency losses from the shock that accompanies abrupt transition from supersonic to subsonic flow. Because the losses from a shock are exponentially related to the pre-shock Mach number, a Mach number approaching 1.0 or lower can greatly reduce or even eliminate the shock losses in an ejector system. Still further, kinetic energy losses can occur as the refrigerant vapor is accelerated by the working fluid in the ejector.